Diesel engines may use re-ingesting burnt exhaust gases to increase fuel economy and reduce emissions. For example, an exhaust gas recirculation (EGR) system may be used to recirculate exhaust gases from the exhaust manifold to the intake manifold. Such operation can displace fresh air and lower oxygen concentration in the cylinder, as well as reduce formation of NOx during combustion.
In some engine configurations that have a turbocharger, both a low pressure and high pressure EGR system may be used. For example, a high pressure (HP) EGR loop from the exhaust manifold (upstream of the turbine of turbocharger) to the intake manifold (downstream of the compressor of the turbocharger), may be used. In addition, a low pressure (LP) loop from downstream of the turbine to upstream of the compressor may also be used. See, for example, U.S. Pat. No. 6,863,058.
The inventors herein have recognized a disadvantage with such an approach. Specifically, in some cases, the HP EGR rate may be estimated based on a mass airflow sensor located upstream of the HP EGR inlet in the intake manifold. However, when using both a HP and LP EGR loop, this reading is typically biased by EGR from the LP EGR system, if both EGR systems are active. As such, degraded estimation, and thus control, of the HP and LP EGR system can result.
Further, the inventors herein have also recognized that when using a dual EGR loop, the interaction between the two EGR flows may cause inadvertent conditions to exist, such as total EGR flow becoming too great or too small. For example, if insufficient pressure exists across the EGR loops, insufficient EGR may be obtained. Likewise, transient conditions may exist where total EGR becomes greater than that desired.
At least some of the above issues may be addressed by a system for a diesel engine having an intake manifold and an exhaust manifold, comprising of a turbocharger coupled between the intake and exhaust manifolds of the engine; a low pressure exhaust gas recirculation system with a first end coupled to the exhaust manifold downstream of the turbocharger and a second end coupled to the intake manifold upstream of the turbocharger, said low pressure exhaust gas recirculation having a first valve coupled thereto for regulating flow; a high pressure exhaust gas recirculation system with a first end coupled to the exhaust manifold upstream of the turbocharger and a second end coupled to the intake manifold downstream of the turbocharger said low pressure exhaust gas recirculation having a second valve coupled thereto for regulating flow; a first mass airflow sensor coupled in the engine intake manifold upstream of said second end of said low pressure exhaust gas recirculation system; and a second mass airflow sensor coupled in the engine intake manifold downstream of said second end of said low pressure exhaust gas recirculation system and upstream of said second end of said high pressure exhaust gas recirculation system.
In this way it is possible to provide a system that can accurately control both high and low pressure EGR flows, even when both systems are concurrently active. In one example, this is provided by the ability to estimate low pressure EGR flow independently of HP EGR flow via use of a sensor located in the intake system upstream of the low pressure EGR inlet, in addition to other information. In another example, two mass airflow sensors may be used to differentiate high and low pressure EGR flow. Still other examples are possible, as described below herein.
In another embodiment, at least some of the above issues may be addressed by a method for controlling a diesel engine having an intake manifold and an exhaust manifold, the engine further having a turbocharger coupled between the intake and exhaust manifolds of the engine, a low pressure exhaust gas recirculation system with a first end coupled to the exhaust manifold downstream of the turbocharger and a second end coupled to the intake manifold upstream of the turbocharger, said low pressure exhaust gas recirculation system having a first valve coupled thereto for regulating flow, a high pressure exhaust gas recirculation system with a first end coupled to the exhaust manifold upstream of the turbocharger and a second end coupled to the intake manifold downstream of the turbocharger said low pressure exhaust gas recirculation system having a second valve coupled thereto for regulating flow, a first throttle coupled in the intake manifold between said second end of said high pressure exhaust gas recirculation system and said second end of said low pressure exhaust gas recirculation system, and a second throttle coupled upstream of said second end of said low pressure exhaust gas recirculation system, the method comprising; adjusting said first valve and said first throttle to regulate flow in the high pressure system; and adjusting said second valve and said second throttle to regulate flow in the low pressure system.
In this way, it is possible to use coordinate control of two EGR flow valves and two throttles in the intake manifold to achieve improved overall operation. For example, sufficient EGR flow from respective high and low pressure systems may be achieved under varying operating conditions.